1. Field of the Invention
The present invention relates to a reactive sputtering method, and more particularly to a reactive sputtering method that can be suitably used for formation of optical thin films, such as an antireflection film and a dielectric multi-layer mirror, usable in optical components for a visible wavelength range and an ultraviolet wavelength range, for example.
2. Description of the Related Art
Conventionally, when optical thin films, such as an antireflection film and a mirror, are formed, a vacuum evaporation method is chiefly used in which a film-forming material is heated in a vacuum using an electron beam or the like, and the material is evaporated and deposited on a substrate.
In general, an antireflection film, a mirror and the like include multi-layers or the like formed of either of a low-refractive-index material, such as a magnesium fluoride (MgF2), and a high-refractive-index material, such as a zirconium oxide (ZrO2), a tantalum oxide (Ta2O5) and a titanium oxide (TiO2), or a combination thereof, respectively. The layer structure, film thickness and the like are variously adjusted in accordance with optical performances required.
Although a vacuum evaporation method is a film-forming method typically conducted in a simple apparatus is simple, in which a film can be formed on a large-area substrate at a high rate, and which provides excellent productivity, the vacuum evaporation technique has the following disadvantages. It is difficult to control the film thickness with high precision, and develop an automatic producing apparatus. When the film formation is performed under a condition at a low substrate temperature, the strength of a film is likely to become insufficient and a film is liable to be easily damaged. Further, a degree of close contact between a film and a substrate is low.
In recent years, the demand for productivity efficiency has increased. Accordingly, also in the field of such optical thin films, there is an increasing demand for a coating by a sputtering method that is more advantageous in simplification or decrease in consumption energy of a process, stability of quality, film quality (a degree of close contact and a film strength), and so forth, as compared with a vacuum evaporation method.
When a dielectric thin film of an oxide, such as a zirconium oxide (ZrO2), a tantalum oxide (Ta2O5), a titanium oxide (TiO2) and an aluminum oxide (Al2O3), is formed by the sputtering method, a high-refractive-index thin film having a low absorptivity can be readily obtained. In the formation of the oxide thin film, however, the sputtering rate is very slow. In the event that a partial pressure of oxygen is reduced and an applied electrical power is increased to increase the sputtering rate, a metal-like film having a large absorptivity is inevitably formed and it is quite difficult to obtain a low-absorptivity thin film stably and rapidly.
Further, it is not easy to form a low-absorptivity thin film of a metal fluoride, such as MgF2 and AlF3, whose refractive index is low, for example below 1.45, which is an important thin-film material that largely influences optical performances of a multi-layer optical thin film, and which is a low-absorptivity material in a range from the visible wavelength to the ultraviolet wavelength.
For example, Japanese Patent Application Laid-Open No. 5(1993)-243155 (JPLO-05-243155) discloses a method of forming such a thin film using a sputtering method. In this method, a sputtering gas is uniformly introduced into a place near a target and a reactive gas is uniformly admitted to a place near a substrate to perform a reactive sputtering.
Further, Japanese Patent Application Laid-Open No. 5(1993)-320891 (JPLO-05-320891) discloses a method in which a sputtering gas is introduced through a small hole formed in a metal target, and a reactive gas is introduced through a ring-shaped conduit to perform a reactive sputtering.
In the event that an Al2O3 thin film most generally used as an optical thin-film material is formed by a sputtering method, a metal Al is used as a target material and a gas, such as O2, is admitted as a reactive gas to form a thin film by a reactive sputtering method.
In the method of JPLO-05-243155, the gas is introduced uniformly from peripheries of the target and the substrate to make the reactivity uniform.
In the method of JPLO-05-320891, the sputtering gas is admitted uniformly from the target provided with an admitting hole to unify the reactivity.
In the above-discussed methods, however, a DC sputtering cannot be stably performed since the reactive gas reacts on a surface of the target. Further, problems of a slow sputtering rate and so forth occur. Moreover, it is difficult to stabilize a surface condition of the target and stabilize the sputtering rate.